When you set a limit of a specific shape and measurement, you've increased the difficulty of the search exponentially. I would relax both of these to open up your choices, at the very least, the exact measurements.
Hi there good luck with your hunt!! I love Montana sapphires (as everyone knows...I am new to gems but they are so unique) some good examples of yellow ones can be found on gemfix.com
This one has tempted me forever with good colour and excellent faceting. It isn't the typical spessartite colouration, showing more red but it is also easy to set, being a calibrated size. http://www.thegemtrader.com/Jan%2009%20Spessartite%20Page.htm
http://www.gemfix.com/sapphire_montana.html - sapphire_montana_304. 7.4 mm round on left side of page. Is $1800 high for a montana sapphire? Looks like it has incredible sparkle.
I can't comment at all about the sapphires as I would defer to Chrono for her expertise ALWAYS! However the setting looks very similar to another PS'er who had theirs made with an oval yellow diamond by Leon Mege.